Metal pipe having a quadrangular cross section, i.e. quadrangular pipe, is used in various applications. For example, quadrangular pipe is used as a structural member in a vehicle, a building, or a large container. Such structural members are required to have resistance against impact, or impact resistance.
For example, WO 2005/058624 (Patent Document 1) discloses a metal pipe to be mounted on the vehicle body of an automobile with both ends supported thereon to provide impact resistance. This metal pipe has a bent portion that runs its entire length or just a portion of its length. The metal pipe is disposed such that the outer side of the bent portion is generally oriented toward the direction of a possible impact applied to the vehicle body. This metal pipe has better impact resistance than a reinforcement member made of straight pipe to reinforce the vehicle body.